mushroom bodies in ants
Mushroom bodies are a brain structure in insects which is assumed to be involved
in context dependent learning and memory processes. The image above shows a
reconstruction of the brain of a worker ant (frontal view) and enlarged just
one mushroom body from the same reconstruction. In ants, bees and wasps the
mushroom bodies have a region called calyx (here red and yellow), into which
visual (yellow) and olfactory (red) information is known to project. Cells which
do not have branches outside of the mushroom bodies (intrinsic cells or Kenyon
cells) receive this information and transmit it to the lobes (blue). The lobes
get their shape from the branching pattern of the Kenyon cells: most of them
have two branches, one into the vertical lobe (alpha lobe) and one into the
medial lobe (beta lobe). But within the lobes there are also many other cells
which receive input from the Kenyon cells or synapse onto those.
Ants are predominantly olfactory insects, which is evident in the large antennal lobe, the neuropil which receives input from the chemoreceptors on the antenna. But also in the mushroom body the importance of olfaction is evident. The neuropil in the calyx which receives olfactory input from the antennal lobes (lip: red) is clearly larger than the neuropil which receives visual input from the optic lobes (collar: yellow),
At least the carpenter ant has a large number of Kenyon cells which are different from those in the honey bee in that they are bimodal, they appear to be postsynaptic to visual and olfactory input. The arborizations of a sinlge cell of this type is shown in the image below (top left) as well as those of a group (top right). While this cells have clearly arborizations in the lip of the mushroom body calyx, they are restricted to a certain part and do not get input from another part of the lip, indicating that those parts may have different functions or modalities.
There are other Kenyon cells which have their cell bodies outside of the calyx cup, clawed Kenyon cells (see Strausfeld 2002 for honey bees). As mass fills indicate there are not many of those cell types which receive visual input from the collar (co), most of those cells have an neurite which goes through the collar without forming any synapses there (bottom left and right). All this emphasises the importance of olfactory information over visual information in ants. Unlike in honey bees at least the majority of clawed Kenyon cells appears to have branches in both, the medial as well as the vertical lobe.
The organization of the axons in the mushroom bodies lobes appears to correspond to that in honey bees. We have not yet seen evidence that the concentric organization of the calyx is maintained in the lobes as claimed by Goll (1967, left), rather the arrangement appears to be transformed in a manner comparable to that found in the honey bee (right). A concentric arrangement in the lobes should in addition be evident in a corresponding morphology of extrinsic neurons, which has not been observed. The pattern of six gross morphological layers in the ant mushroom body does however, not appear to coincide closely with the three concentric layers in the calyx (middle). The first three cross-morphological layers which contain class I Kenyon cells (spiny or bunched cells with cells bodies inside the calyx cup) are all of comparable width, though the basal ring in carpenter ants is very small and the axons form a very thin layer in the lobes (dorsalmost for the vertical lobes). The three much thinner layers which contain class II Kenyon cells (clawed Kenyon cells with cell bodies outside the calyx cup) are much more narrow but also of comparable width to each other. Those layers can be found in the vertical as well as the median lobes, indicating that the majority, if not all, fibers of the class II Kenyon cells have projections into both lobes. The fibers in the median lobes are, however, clearly shorter than those of the class I Kenyon cell, which part of the median lobe leads backwards at the midline. On possibility is that cells extrinsic to the mushroom bodies are responsible for this observed pattern, and while the modalities of those cells may differ between bees and ants the cell types may not be that different.
references to some old mushroom body literature